Pollution is a pretty fundamental concept in the game. Pollution
has several negative side effects, including a reduced life
expectation, a lower Mayor Rating and a lower desirability and thus
abandonment. Therefore, you'll generally want to avoid pollution in
your city.

However, because in practice this often isn't possible, it's
best to rather confine pollution to a specific location. As
pollution doesn't spread beyond the city borders, placing polluting
industries and structures at a corner is an effective strategy at
reducing pollution.

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Air pollution is caused by quite many objects, including dirty
and manufacturing industry, most power plants, airports, and also
traffic. Note that the spread of air pollution is affected by the
terrain, so building industries behind a mountain range, for
example, will restrict the polluted air from moving beyond the
mountains. You can use that to your advantage in God Mode by
terraforming an industrial zone entirely enclosed by mountains.

Air pollution can be reduced by parks as well as trees (both the
trees from God Mode and manually planted trees work fine). Some of
the rewards, such as the cemeteries, are also effective ways of
limiting air pollution. There are also a few ordinances that help
reduce air pollution.

Water pollution is caused by industry, and very much by farms.
Unlike air pollution, it isn't too much of a problem (although it
has the same negative effects); note however that if the water
pollution around your water towers or pumps is too high, they will
be shut down, therefore build your water towers and pumps away from
sources of water pollution by any means.

The only way to combat water pollution in the game is to build a
water treatment plant, however due to their high cost and low
capacity, it is not advisable to build them except in very rare
cases.

Even though it's not noted as a separate type of pollution in
the game, traffic pollution or traffic noise can also have negative
side effects. Naturally, any ways of reducing the car traffic will
lower traffic noise. Also try to route traffic through commercial
areas, which are positively affected by high traffic.

Besides a few oddball rewards, there are two main sources of
radiation: the Toxic Waste Dump business deal,
and the meltdown of a nuclear power plant. Radiation is an absolute
killer to desirability, nothing at all will grow in an irradiated
area. While the radiation of a toxic waste dump will disappear soon
after the building is demolished, radiation caused by a meltdown
will stay for decades, and as there isn't a way to clean up
radiation, after a meltdown it's usually best to just start over
the city.